Hitherto-unknown lymphatic vessels discovered in the brain: Immune system might be controlling our social behavior

As part of a new research, scientists from the University of Virginia have discovered a hitherto-unknown series of lymphatic vessels that connect the brain to our immune system. The findings, according to the team, could help enhance our understanding of the relationship between the immune system and the human brain, particularly how the former controls the latter’s functions as well as our behavior in general. Speaking about the project, Jonathan Kipnis, the study’s chief researcher, said:

I really did not believe there were structures in the body that we were not aware of. I thought the body was mapped.

The discovery, which originally appeared in the Nature journal last year, managed to garner interest only recently, when the scientists revealed that the immune system could very well be influencing our social behavior via the newly-found vessels. As the team explains, the lymphatic system consists of a network of vessels that in turn carry immune cells, such as white blood cells, to different parts of our body.

In doing so, it acts as bridge between the bloodstream and the tissues, assisting in the removal of waste, including dead blood cells. Up until now, researchers were of the opinion that these vessels were completely absent in the brain, an organ that they believed had no direct connection whatsoever to the immune system. This, coupled with the blood-brain barrier (BBB), was thought to protect the brain from the attacks of pathogens.

In June of last year, however, the team from Virginia University came across these lymphatic vessels in the meninges region of the brain. For the research, the scientists examined the whole meninges of a mouse using a powerful microscope. Instead of cutting it into smaller parts, the murine meninges was studied in its entirety. The team found numerous immune cells extending across the meninges in a somewhat vessel-like pattern.

Closer inspection revealed that the structures were actually lymphatic vessels. To observe how the vessels transport immune cells and blood, the researchers introduced a special dye into an anesthetized mouse. The vessels, according to them, were found to carry the white blood cells from the cerebrospinal fluid, via the veins present in the sinuses, to the deep cervical lymph nodes, thus establishing a direct link between the brain and the body’s immune system.

Autopsies of human brains revealed similar structures in the meninges; a discovery that the scientists believe could completely change our understanding of human behavior. At present, there are a lot of questions that need to be answered. For instance, how are these lymphatic vessels organized in humans? The team is currently trying to figure out what their exact function is. Such information, as the researchers point out, could pave the way for new treatment options for a number of diseases, including Alzheimer’s and schizophrenia. Kipnis added:

In Alzheimer’s, there are accumulations of big protein chunks in the brain. We think they may be accumulating in the brain because they’re not being efficiently removed by these vessels… We believe that for every neurological disease that has an immune component to it, these vessels may play a major role.

As part of the current research, the scientists also demonstrated the role the newly-found lymphatic vessels play in determining our social behavior. According to them, turning off even a single immune system molecule was found to greatly affect the mouse’s behavior, causing them to even stop interacting with other mice. Kevin Lee, the head of the university’s Department of Neuroscience, was reported saying:

I just said one sentence: ‘They’ll have to rewrite the textbooks’. There has never been a lymphatic system for the central nervous system, and it was very clear from that first singular observation – and they’ve done many studies since then to bolster the finding – that it will fundamentally change the way people look at the central nervous system’s relationship with the immune system.